This invention relates in general to an apparatus and method for estimating position of a rotor of a variable reluctance drive operating without a shaft position sensor and, more specifically, to estimating rotor position from the inductance characteristics of unenergized stator phases.
Although they have been known for some time, interest in switched reluctance motor (SRM) drives has recently revived. Compared to conventional induction and synchronous motor drive systems, the SRM drive is simple in construction and economical. In addition, the converter which applies power to the SRM machine requires fewer power devices and, therefore, is more economical and reliable. In view of these advantages, the switched reluctance motor drive system provides an attractive alternative to conventional drive systems and it is expected to find wide popularity in industrial applications.
Switched reluctance motors conventionally have multiple poles or teeth on both the stator and rotor, i.e., they are doubly salient. There are phase windings on the stator but no windings or magnets on the rotor. Each pair of diametrically opposite stator poles is connected in series to form an independent phase of the multiphase switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to the stator phase windings by a converter using unidirectional current switching elements such as thyristors or transistors.
The switched reluctance drive operates by switching the stator phase currents on and off in synchronism with rotor position. By properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained.
Usually, the desired phase current commutation is achieved by feeding back a rotor position signal to a controller from a shaft position sensor, e.g. an encoder or resolver. For cost reasons in small drives and reliability reasons in larger drives and to reduce size, weight, and inertia in all such drives, it is desirable to eliminate this shaft position sensor.
To this end, various approaches have previously been proposed for indirect rotor position sensing by monitoring terminal voltages and currents of the motor. One such approach, referred to as waveform detection, depends upon back electromotive forces and is therefore unreliable at low speeds and inoperative at zero speed.
Commonly assigned U.S. Pat. Nos. 4,611,157 and 4,642,543 describe work that has been done on dynamically stabilizing switched reluctance drives by feeding back the average D.C. link current rather than shaft position. Such controllers are limited by the average nature of their feedback information and by the tendency to jitter at start-up. While useful for fan and blower type applications, these controllers are not applicable to servo type applications where precise speed and/or position control is required.
U.S. Pat. No. 4,520,302 for "Stepping Motors and Drive Circuits Therefor" recognizes that the inductance of a phase winding is dependent on rotor position and varies substantially sinusoidally from a maximum to a minimum as the rotor advances over a pole pitch. According to this patent, the variation of inductance causes corresponding variation in certain characteristics of phase current flow which can be monitored to derive an indirect indication of rotor position. The current flow through an energized or unenergized phase winding may be monitored. In the case of a chopper type drive circuit, the characteristic of current flow that is measured may be current rise time, current decay time or the chopping frequency. Although various implementations are suggested in this patent, they all appear to involve a search for a known, e.g. minimum, inductance value based on measured phase current changes and any ambiguity in detected position for the target inductance is eliminated by considering whether the current flow characteristic being monitored is increasing or decreasing with rotor position. (Reference column 6, lines 62-65 and column 8, lines 12-19.) This ambiguity resolving approach assumes that the motor is moving in a given direction. Accordingly, it would not appear to be effective when the motor is starting from standstill. This latter limitation is particularly significant in servo drive systems where one cannot tolerate having the drive jiggle when it is started.
The indirect rotor position estimating and feedback approach of U.S. Pat. No. 4,520,302 is further discussed in a paper entitled "Detection of Rotor Position in Stepping and Switched Motors by Monitoring of Current Wave Forms" published in the IEEE Transactions on Industrial Electronics, Vol, IE-32, No. 3, August 1985 at pages 215-222. In an application of their approach to a ministep drive, the authors of this paper recommend monitoring of the chopping characteristic of phase current in both unexcited phases of a four-phase motor. The authors state that this is important "because the slope of each inductance/position characteristic approaches zero at one end of the position range." At intermediate points in the range, this prior approach apparently relies upon the predetermined direction of motor rotation to resolve ambiguities with respect to rotor position.
In switched reluctance motors, unlike ministep drives, there are not always two unexcited stator phases which can be monitored. Further, there are occasions when it is essential to ascertain rotor position without knowing, or being able to assume, the direction of rotor rotation, e.g. when starting a servo drive. A need thus persists for a method and implementing apparatus which can accurately estimate instantaneous rotor position in an SRM, irrespective of motor speed or direction and without resort to a rotor position sensor.